In downhole tubular strings, hydraulic pressure may be used to actuate various components for example, packers may be pressure set, sleeve valves may be provided that are hydraulically moveable to open ports.
Although hydraulically actuable components are useful, difficulties can arise when there is more than one hydraulically actuable component to be separately actuated. In a system including pressure set packers and sleeve valves for tubular ports, difficulties have occurred when attempting to open the sleeve valves after the packers have been set.
Also, difficulties have occurred in strings where it is desired to run in the string with all ports closed by hydraulically actuable sleeve valves and then to open the sleeves at a selected time. If one port opens first, it is difficult to continue to hold pressure to move the sleeves from the remaining ports.